Since a conventional whole-body X-ray computed tomography apparatus carries out imaging for a patient who lies on the bed, image analysis and dynamic observation is difficult to carry out because of the patient's own weight being applied to the patient's joints and organs. An X-ray computed tomography apparatus arranged in such a manner that its gantry is placed in parallel with the ceiling and the floor so that the gantry can be driven in a vertical direction (hereinafter, a standing-state radiography X-ray CT apparatus) is therefore clinically useful to facilitate image analysis and dynamic observation. The standing-state radiography X-ray CT apparatus can carryout imaging of a subject in a standing state or a sitting state by driving the gantry in a vertical direction. The driving of the gantry in the vertical direction is performed by synchronous control on mechanism units which are respectively provided at two columns that support the gantry from both sides thereof.
When driving the gantry in the vertical direction in the standing-state radiography X-ray CT apparatus, regardless of the synchronous control on the mechanism units provided at two columns, a misalignment which is a difference in the height of the gantry from the floor between the left and right columns may occur due to malfunction of the mechanism units or physical factors, such as an object being caught between the gantry and the column. If imaging is carried out with a significant misalignment which is caused when adjusting an imaging position, a reconstruction image will be affected by the misalignment. In such a case, imaging needs to be performed once again and an amount of X-rays to which a subject is exposed will be increased. Since the axes of the two columns are misaligned because of the misalignment, an intended operation for the gantry may not be properly carried out. In such a case, there may be harm to a subject, such as a subject being caught between the gantry and the column, or a malfunction of the apparatus. Since the gantry of the standing-state radiography X-ray CT apparatus is very heavy, a misalignment may cause serious harm.